Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used recently for various types of optical materials, particularly eyeglass lenses. Optical materials, particularly eyeglass lenses, are specifically required to have, as main properties, low specific gravity, high transparency and low yellowness, and as optical properties, high refractive index and high Abbe number. Recently, a polymerizable composition for optical materials, in which an inorganic compound having a sulfur atom and/or a selenium atom is blended with a polyepisulfide compound for the purpose of achieving a high refractive index and a high Abbe number, has been proposed (e.g., Patent Document 1).
Though a high refractive index can be achieved by an optical material obtained by polymerizing and curing such a composition, in many cases, an inorganic compound having a sulfur atom and/or a selenium atom is solid at ordinary temperature and has a low solubility, and therefore, there are problems in that precipitation may occur when providing a composition and the compound may be dissolved insufficiently when the concentration thereof is high.
For this reason, a technique in which an inorganic compound having a sulfur atom and/or a selenium atom and a sulfur-containing organic compound such as a polyepisulfide compound that can react with the inorganic compound are subjected to a prepolymerization reaction in advance has been proposed (see Patent Document 2). However, when the content of the inorganic compound having a sulfur atom and/or a selenium atom is increased, the viscosity of a polymerizable composition becomes too high, and it becomes difficult to carry out usual injection and polymerization operations such as filtration and mold injection. Therefore, for the purpose of reducing the viscosity at the time of the prepolymerization reaction, the addition of a compound having one SH group (see Patent Document 3), a compound having one or more NH groups and/or NH2 groups (see Patent Document 4) or a compound having one or more disulfide bonds (see Patent Document 5) has been proposed. However, in the case of a composition in which the content of an inorganic compound having a sulfur atom and/or a selenium atom is 10 parts by mass or more, even when an additive for reducing the viscosity of the composition is added, the viscosity may be increased, for example, in the case where a prepolymerization reaction is performed excessively and/or the temperature after the preparation of the composition is too high, or the viscosity after about 3 hours, which is the injection time usually required for producing an optical material industrially, may be significantly increased. As a result, it becomes difficult to carry out usual injection operations such as filtration and mold injection, and there are problems in that increasing in size of apparatuses for filtration and injection and shortening of the injection line are required. Moreover, for the purpose of improving the productivity of optical materials, a polymerizable composition for optical materials having a lower viscosity, which makes it easier to carry out filtration and injection operations, has been desired.
Further, since the prepolymerization reaction of the aforementioned reference is usually performed at 50° C. to 70° C., it is necessary to cool the composition to about room temperature which is the temperature of initiation of polymerization after mold injection, and the cooling step is required after the completion of the prepolymerization reaction. For increasing a reaction scale, unless increasing in size of a cooling apparatus, introduction of a cooling coil into a reaction apparatus, etc. are carried out, the time required for cooling increases or changes in each case. This is also the problem. For this reason, a method for producing a polymerizable composition for optical materials which can be subjected to a prepolymerization reaction at a reaction temperature near room temperature is desired.
In Patent Documents 1 and 2, an optical material having a high refractive index can be obtained, but mold release characteristics at the time of releasing it from a mold after polymerization and curing may be poor. In the case of optical materials having a complicated shape, in particular, an optical lens, the smaller the radius of curvature of the lens is, the poorer the mold release characteristics tend to be, and it is extremely difficult to improve mold release characteristics of minus-power lenses of −15.0 D or more. In the case of poor mold release characteristics, the production time may be increased, a defect of an optical material and/or a mold may be generated, and it may become impossible to use the optical material and/or the mold. Thus, the production may be affected thereby. In order to improve mold release characteristics of optical materials, a mold release agent is generally used, and external mold release agents, which are applied to a mold and used, and/or internal mold release agents, which are added to a monomer and used, are known (see Patent Document 6).
However, the method of applying a mold release agent to a mold is very complicated, and there are problems in that the surface of an optical material becomes rough due to an external mold release agent and turbidity is generated in an optical material. Further, also in the case of internal mold release agents, there are problems in that the addition of just a small amount of an internal mold release agent generates turbidity in an optical material and a prepolymerization reaction and/or polymerization reaction rate are affected thereby to generate many striae in the optical material. Therefore, an optical material which has good mold release characteristics and has substantially no striae without the above-described influences and a method for producing the same have been desired.
Further, the prepolymerization reaction of Patent Documents 1 and 2 is usually performed at 50° C. to 70° C., and it is necessary to cool the composition to about room temperature which is the temperature of initiation of polymerization after mold injection. However, since the preliminary reaction temperature is high, there is a problem in that the viscosity of the polymerizable composition is elevated by the progress of side reaction, significant reduction in the temperature due to cooling or the like. When the temperature after cooling is set at a high temperature in order to suppress the viscosity elevation, there are problems in that the pot life is shortened and striae are generated due to the difference between the temperature after cooling and the temperature of holding at an early stage of the polymerization step. Therefore, a production method in which a polymerizable composition for optical materials comprising a sulfur-containing organic compound such as a polyepisulfide compound and an inorganic compound having a sulfur atom and/or a selenium atom can be subjected to a prepolymerization reaction at a reaction temperature near room temperature has been desired.